Carrier wave telephony system



Swt. 24, 1940.

T. S. SKILLMAN CARRIER WAVE TELEPHONY SYSTEM Filed Dec. 17, 1938 3L 61101 25 3P5 if:

5 n 10 if L 1! GENERATOR fwbwru? TlSISkz'ZZm/z -Patented Sept. 24, 1940UNITED STATES oaaamn WAVE mmrnoiw SYSTEM 1 Thomas Samuel -Skillman,Eindhoven, Netherl b, asaignor, by mesne cuts, to Hartford National Bankand Trust Company, Hartford, Conm, as trustee Application December 11,1938, Serial N... 246,459

, p In Germany December 27, 1937 2 Claims. (o 119-1) My inventionrelates to a signal-receiver for carrier wave telephony systems.

In carrier-wave telephony systems it is known to transmit signals, suchas calling currents or 5 selecting impulses, by transmitting thecarrierj wave which is suppressed during transmission of theconversation. These signals are received on the receiving side by asignal receiver which 1' a comprises an oscillatory circuit tuned to thecarrier-wave frequency and across which the carrier-wave produces avoltage which, after rectification, controls a relay operating thesignalcurrent circuit. 4

To prevent false operation of the signal-current circuit duringtransmission of the conversation, the oscillatory circuit must have sucha high selectivity that only the carrier-wave fre-' quency, but not. theside-band frequencies occurring during transmission of the conversation,sets up a voltage at the oscillatory circuit. As

the lowest audio-frequency to be transmitted is about 200 cycles persec., the corresponding.

' side-band frequency lies at a distance of only about 200 cycles persee. from the carrier-wave. The higher the carrier-wave frequency, themore diificult his to provide an oscillatory circuit which is capable ofdistinguishing between two frequencies which differ by only 200 cyclesper six The object of the present invention is to overcome the abovedifliculty, and for this purpose I superpose on the carrier wave emittedduring the signal transmission and in the signal-receiver, an auxiliaryfrequency which differs only slightly from the carrier-wave frequency.The difference in frequency so produced is sieved out by means of afilter or an oscillatory circuit, is rectified, and the rectifiedcurrent is used to control the relay which controls the signal-currentcircuit.

As the auxiliary frequency can be made only.

slightly different from. the carrier-wave frequency, thedifference-frequency produced is low,

and it is possible without additional means to increase the selectivityof the oscillatory circuit or filter tuned to this difierence frequencyto such a degree that the lowest side-band frequency, which differs onlyby about 200 cycles per sec., does not cause undesirable responses inthe signal-current circuit.

The auxiliary frequency is preferably taken from a carrier-wavegeneratorof another channel, for instance a generator whose frequencydiffers, for instance by about 8 kilocycles per sec. from the receivedcarrier-wave frequency.

In this case the difference-frequency is also 8 kilocycles per second.

I am aware of the fact that in known signalreceivers including'anoscillatory circuit tuned to the carrier-wave, it has been proposed toavoid 5 undesirable signal responses by making the carrierwave amplitudemany times larger than the disturbing side-band frequencies. However,this measure has the drawback that the transmitting, receiving andintermediate amplifiers may 10 be over-controlled, thereby producingnon-linear 'cross talk and noises in the other channels of thecarrier-wave system. When using the signal I receiver according to theinvention this drawback does not occur, because the carrier-wave amplil5tude is not greater than'the average side-band amplitude need be duringthe transmission of the conversation.

' In order that the invention may be clearly understood and readilycarried into effect, I shall 20 describe the same in more detail withreference to the accompanying drawing, in which the single figure is aschematic diagram of a portion of a carrier-wave telephone systemcomprising a signal receiver according to the invention. 25

In the system shown in the drawing, in which the signals are transmittedby the emission of a carrier wave which is suppressed during thetransmission of conversation, the conversation to be transmitted isreceived through the lines l 30 and is supplied in the usual mannerthrough a differential transformer 2 with a diagrammatically-indicatedbalancing network 3, to a modulator 4. The speech currents are modulatedby modulator 4 on a carrier-wave of a frequency 35 j, which is producedby a high-frequency generator i.

Modulator 4 is so designed that the carrierwave produced by generator 5is suppressed, and that there will be only two side-bands available 40in the output circuit thereof. After passing through band pass-filters45 and 6 one of these side-bands is suppressed and the other side-bandis supplied to a transmission line I. Additional similar transmissionchannels may of course be 45 connected to line I.

For signaling there is provided a relay 8 which has its coil connectedthrough a single-pole switch 9 to a suitable voltage supply (not shown),and has two single-throw, normally-closed 50' armatures l0 and IIadaptedto engage fixed contacts l2 and I3 respectively. The heels ofarmatures II and II are connected to the terminals of generator 5, andthe contacts I: and ii are connected to conductors l4 and I! whichinter- 55 connect the band pass filters 45 and 6. Thus, by closing andopening switch 9, call or selective impulses supplied by generator 5 areimpressed upon the line I.

The carrier-wave conversation received through line I6 is separated fromthat of the other channels (not shown) by means of bandpass filters lland I8 and is demodulated in a demodulator 20. A carrier-wave offrequency f1, produced by a high-frequency generator I9 is added. Thedemodulator current is amplified by a valve 2| and the amplifiedcurrent, after being transformed by a transformer 22, is supplied aslow-frequency speech current to line Controlled by the anode current oftube 2| is a relay 31 having a single-throw normally-closed armature 38which controls a signal circuit 39.

The signal impulses having the carrier frequency f1, which come in overline l6, are supplied through conductors 23 and 24 and a. transformer 25to the grid circuit of a triode 26, which also comprises a condenser 21shunted by a leakage resistance 28.

Coupled to the anode circuit of tube 26 through a transformer 32 is amixing circuit comprising a rectifier 29, an auxiliary high-frequencygenerator 30 of a frequency f2, and the primary winding of a transformer3|. Connected across the secondary winding of transformer 3| is anoscillatory circuit comprising a condenser 33 and an inductance coil 34,which circuit, in accordance with the invention, is tuned to a frequencyis equal to the difference between the frequencies of generators 30 andI9, i. e., f3=f2j1.

The voltage set up across inductance coil 34 is rectified by a rectifier35 and applied across a resistance 36 in the grid circuit of tube 2|.The voltage drop thereby produced in the grid circuit of tube 2| is insuch a direction that the anode current of the tube is greatlysuppressed or reduced to zero. As a result, relay 31 is deenergized anda signal impulse is produced in a signal circuit 39.

As referred to above the auxiliary frequency {2 is preferably taken froma carrier generator of another channel for instance from the generator 5of the sending channel. In this case it is only necessary to remove thegenerator 30 and to connect the mixing circuit comprising the rectifier29 to the generator 5.

While I have described my invention with reference to a specificexample, I do not wish to be limited thereto but desire the appendedclaims to be construed as broadly as permissible in view of the priorart.

What I claim is:

1. In a carrier-wave telephony system having a main. channel fortransmitting suppressed carrier speech-modulated waves from atransmitter, a receiver connected to said transmitter by a transmissionline and comprising a demodulator, a signal circuit, a side channelderived from said main channel at a point before the demodulator andresponsive to additional signals transmitted by transmission of thecarrier frequency which is suppressed during the transmission of saidspeech-modulated waves, means for selecting said carrier frequency,means for superposing on said carrier frequency an auxiliaryoscillation. of a frequency slightly different from the carrierfrequency to produce a difference frequency, means for filtering out thedifference frequency, and means to control the signal circuit by saiddifference frequency.

2. In a carrier-wave telephony system having a main channel fortransmitting suppressed carrier speech-modulated waves from atransmitter, a receiver connected to said transmitter by a transmissionline and comprising a demodulator, a signal circuit, a side channelderived from said main channel at a point before the demodulator andresponsive to additional signals transmitted by transmission of thecarrier frequency which is suppressed during the transmission of saidspeechmodulated waves, means for selecting said carrier frequency, meansfor superposing on said carrier frequency an auxiliary oscillation of afrequency slightly different from the carrier frequency to produce adifference frequency, said latter means including a carrier-wavegenerator of another channel of the system, means for filtering out thedifference frequency, and means to control the signal circuit by saiddifference frequency.

THOMAS SAMUEL SKILLMAN.

